PCB design of the single-chip control board design principles

Whether it is the layout of the devices on the PCB board or alignment and so on have specific requirements. For example, the input and output lines should try to avoid parallel, so as not to generate interference. Two signal lines parallel alignment is necessary to add ground isolation, two adjacent layers of wiring to try to perpendicular to each other, parallel easy to produce parasitic coupling. Power and ground should be divided as far as possible in two layers perpendicular to each other. Line width, the digital circuit PCB available wide ground line to make a circuit, that is, to form a ground network (analog circuits can not be used in this way), with a large area of copper laying.

The following is a description of the principles and some details that need to be taken into account in the design of the microcontroller control board.

1.Component layout

In terms of the layout of components, the components related to each other should be placed as close as possible, for example, the clock generator, crystal, CPU clock input are prone to noise, they should be placed closer to some. For those devices prone to noise, small current circuits, high current circuit switching circuits, etc., should be as far away as possible from the logic control circuit and storage circuit of the microcontroller (ROM, RAM), if possible, these circuits can be made into a separate circuit board, which is conducive to anti-interference, improve the reliability of the circuit work.

2.Decoupling Capacitor

Try to install decoupling capacitors next to key components, such as ROM, RAM and other chips. In fact, PCB board alignments, pin connections and wiring may contain large inductive effects. Large inductors may cause severe switching noise spikes on the Vcc alignment. The only way to prevent switching noise spikes on the Vcc alignment is to place a 0.1uF electronic decoupling capacitor between VCC and power ground. If a surface mount component is used on the PCB, a chip capacitor can be used directly adjacent to the component and fixed on the Vcc pin. It is best to use porcelain chip capacitors, this is because this capacitor has a low electrostatic loss (ESL) and high frequency impedance, in addition to the dielectric stability of this capacitor over temperature and time is also very good. Try not to use tantalum capacitors because of its high impedance at high frequencies.

The following points need to be noted when placing decoupling capacitors.

(1) Connect an electrolytic capacitor of about 100uF across the power input of the PCB, or better yet, a larger capacitance if size allows.

(2) In principle, a 0.01uF ceramic chip capacitor needs to be placed next to each IC chip. If the board gap is too small to fit, a tantalum capacitor of 1~10 can be placed every 10 chips or so.

(3) For components with weak immunity to interference and large current changes when turned off, and storage components such as RAM and ROM, a decoupling capacitor should be connected between the power line (Vcc) and ground.

(4) The leads of the capacitors should not be too long, especially the high-frequency bypass capacitors should not come with leads.

The following points need to be noted when placing decoupling capacitors. 3. Ground design

In microcontroller control systems, there are many types of grounds, including system ground, shield ground, logic ground, analog ground, etc. Whether the ground is laid out properly will determine the board's immunity to interference. When designing grounds and grounding points, the following issues should be considered.

(1) Logical and analog grounds should be wired separately and not combined, connecting their respective grounds to the corresponding power ground. The analog ground should be as thick as possible during design, and the grounding area of the lead end should be as large as possible. In general, for the input and output analog signals, it is best to isolate them from the microcontroller circuitry via optocouplers.

(2) In the design of the printed circuit version of the logic circuit, the ground should form a closed-loop form to improve the circuit's immunity to interference.

(3)(3) The ground wire should be as thick as possible. If the ground wire is very thin, the ground resistance will be larger, causing the ground potential to change with the current, resulting in an unstable signal level and leading to a decrease in the circuit's immunity to interference. In the case of wiring space allows, to ensure that the width of the main ground line at least 2 ~ 3mm or more, the ground line on the component pins should be about 1.5mm.

(4)Pay attention to the choice of grounding point. When the signal frequency on the board is lower than 1MHz, the influence of electromagnetic induction between the wiring and components is very small, and the loop current formed by the grounding circuit has a greater impact on the interference, so a point of grounding should be used so that it does not form a loop. When the signal frequency on the board is higher than 10MHz, the ground impedance becomes large due to the obvious inductive effect of the wiring, and the loop current formed by the grounding circuit is no longer a major problem at this time. Therefore, multiple points of grounding should be used to minimize the ground impedance.

4. Other

(1) The layout of the power cord in addition to the size of the current to try to thicken the width of the alignment, in wiring should also make the power cord, ground line alignment direction and data line alignment body in line with the wiring work at the end of the ground line will be the bottom of the board without the alignment of the pavement, these methods help to enhance the anti-interference ability of the circuit.

(2) The width of the data lines should be as wide as possible to minimize impedance. The width of the data lines should be at least 0.3mm (12mil), and it would be better if 0.46~0.5mm (18mil~20mil) is used.

(3) Since one vias of the board will bring about 10pF of capacitive effect, which will introduce too much interference for high frequency circuits, the number of vias should be reduced as much as possible when wiring. Then again, too many vias can cause the mechanical strength of the board to be reduced.